1. Field of the Invention
The present invention generally relates to the field of high frequency electronic signal amplifiers, and more specifically to a monolithic microwave integrated circuit (MMIC) field-effect transistor (FET) amplifier.
2. Description of the Related Art
Monolithic microwave integrated circuits on gallium arsenide (GaAs) and other semi-insulating substrates enable active array radar, communications and other systems to be produced which are compact, lightweight, provide high performance and can be manufactured inexpensively on a commercial production basis. A general treatise on MMICs is presented in an article entitled "MMIC Technology: Better Performance at Affordable Cost", in Microwave Journal, April 1988, pp. 135-143.
Distributed circuit arrangements are employed in MMIC amplifiers to increase the power handling capability. This is accomplished by fabricating a plurality of amplifier unit cells on a substrate and interconnecting them in a parallel interdigitated configuration.
An example of such a prior art MMIC amplifier is illustrated in FIG. 1 and generally designated as 10. The amplifier 10 includes a plurality of transversely spaced metal-semiconductor field effect transistor (MESFET) unit cells fabricated in a GaAs substrate, each including doped source, drain and channel regions which are not visible in the drawing. The amplifier 10 further includes a plurality of parallel, elongated drain pads 14 which overlie the drain regions of the MESFET cells in electrical connection therewith. The right ends of the drain pads 14 are interconnected by a drain foot 16 which enables the drain pads 14 to be collectively connected to external circuitry.
A plurality of source pads 18 are interspersed between the respective drain pads 14 in electrical connection with the source regions of the MESFET cells. A plurality of gate fingers 20 are interspersed between the respective drain pads 14 and source pads 18 in electrical connection with the respective channel regions of the MESFET cells, and interconnected at their left ends to a gate foot 22.
Source connection pads 24 and 26 are provided at the upper and lower ends of the amplifier 10, and connected to a ground plane (not shown) on the opposite side of the substrate 12 by electrically conductive vertical interconnects or vias 28 and 30 respectively. The source pads 18 are connected to the source connection pads 24 and 26 by an electrically conductive airbridge 34 having first portions 34a which are ohmically bonded to the source pads 18 and second portions 34b which extend over and are electrically isolated from the drain pads 14 and gate fingers 20.
The amplifier 10 has a common emitter configuration, with the source pads 18 grounded through the vias 28 and 30 and ground plane. An input signal is applied to the gate regions of all of the MESFET cells in parallel through the gate foot 22 and gate fingers 20, and an output signal is taken from the drains of all of the MESFET cells in parallel through the drain pads 14 and drain foot 16.
In this configuration, the total collector current flow through the amplifier 10 is distributed between the MESFET cells. However, due to the architecture of the amplifier 10, the electrical characteristics of the unit MESFET cells are different and the collector currents through the individual cells are also different.
More specifically, the source pads 18 are effectively connected to the source connection pads 24 and 26 in series through the airbridge 34. The source inductance, and thereby the inductive reactance and collector current varies between the unit cells, with the source inductance of the inner cells being higher than the outer cells. For am amplifier with a large number of cells, the source inductance of the inner cells can become excessive, thereby reducing the cutoff frequency of the amplifier.
The series connection of the source pads 18 to the source connection pads 24 and 26 also creates a phase difference between the signals propagating through the individual MESFET cells, thereby reducing the signal combining efficiency of the amplifier 10. The connection of the portions 34a of the airbridge 34 to the source pads 18 requires a relatively large amount of space, thereby increasing the spacing between the gate fingers 20 and further increasing the signal phase variation.
Further drawbacks of the arrangement of the amplifier 10 are that it occupies an inefficiently large amount of space on the substrate 12, and is limited to only high power applications.